1. Field of the Invention
The present invention relates generally to a fiber optic cable having a pre-stressed ribbon stack, and more particularly to a cable having stacked ribbons which are pre-stressed according to a distribution that corresponds to expected thermo-mechanical conditions to occur during the service of the cable.
2. Description of the Related Art
In fiber optic cables with stranded, twisted, or straight ribbon stacks, the corner fibers experience a high level of attenuation, which reduces the cable""s performance characteristics and limits the level of recommended thermo-mechanical loads. As shown in FIG. 4, the comer fibers 41a of the laminated ribbon stack 41 of the fiber optic cable 40 are located the most remotely from the geometrical center of the ribbon stack. Consequently, under mechanical bending and contact stresses from surrounding cable components such as buffer tube 42, the comer fibers 41a usually have maximum stress levels as compared with the centrally located fibers. Moreover, under thermal loading, the thermoplastic material of the buffer tube 42 expands and contracts in both longitudinal and radial directions. Corresponding displacement of the thermally loaded buffer tube 42 places an additional stress on the comer fibers 41a. The attenuation in the comer fibers 41a of the ribbon stack 41 may be reduced by increasing the gap between the comers of the ribbon stack 41 and the buffer tube 42 (i.e., by providing a larger buffer tube). But this approach leads to an undesirable increase in the cable diameter.
It is an object of the present invention to improve the optical performance of the cable through increased thermo-mechanical load resistance of a cable in which a laminated ribbon stack is provided.
It is a further object of the present invention to reduce the attenuation experienced by the comer fibers of an optical fiber ribbon stack provided in a cable, without increasing the diameter of the cable.
These objects are fulfilled by constructing an optical fiber ribbon stack in which some of the optical fiber ribbons are pre-stressed according to a predetermined tension/compression distribution before being laminated together in a stack. This distribution corresponds to expected thermo-mechanical conditions which occur during the service of the cable housing the ribbon stack, and may be linear or non-linear, symmetric or non-symmetric, including anti-symmetric.